Author Topic: Winding connection of transformers in power systems  (Read 679 times)

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Offline Nikos A.

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Winding connection of transformers in power systems
« on: September 23, 2021, 01:59:16 pm »
Hi everyone,

Does anyone know how the transformers are connected in power transmission and distribution systems? I mean star-delta, delta-star, delta-delta etc
In Cyprus after the alternator there is the step-up transformer (11KV/132KV), then the HV step down transformer (132KV/11KV) and finally additional LV transformers for distribution (11KV/400V)

I believe that the secondary of the LV transformer is star connected in order to provide neutral to feeders, but what about the primary? Also what about the rest of the transformers?

Thanks
 

Offline richard.cs

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Re: Winding connection of transformers in power systems
« Reply #1 on: September 23, 2021, 03:53:03 pm »
In the UK at least, 11kV/400V distribution is delta-star to provide a neutral at the secondary and to constrain third harmonic currents to the primary rather than the upstream network. 11 kV neutral is not distributed, but the output winding of an 11 kV source is usually star with the neutral point earthed via a current limiting impedance for fault detection, not sure what the primary is in this case. Very high voltage transformers, e.g. 400kV/132kV tend to be autotransformers, which then requires them to be star/star.

Note that more than one phase relationship is possible with a delta-star transformer. Dyn30 is usual for 11kV/400V in the UK.
 
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Offline Nikos A.

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Re: Winding connection of transformers in power systems
« Reply #2 on: September 23, 2021, 04:14:34 pm »
Thank you richard.cs,

I would expect probably delta-star in all transformers since delta reduces 3rd harmonics as you said and star reduces the phase voltage (so less insulating material and lower manufacturing cost). But in case of delta-star in HV transformers, what about the neutral point? This is grounded right? Because as far as I know there so no neutral in HV trannsmission lines.
 

Offline richard.cs

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Re: Winding connection of transformers in power systems
« Reply #3 on: September 23, 2021, 05:17:35 pm »
What voltage range do you mean by HV? The "standard" meaning over everything >1000V just isn't that helpful here.

The neutral is not distributed, but some means is necessary to centre the voltage of the three phases around earth, whether a star-secondary or a dedicated "earthing transformer". Most systems limit current to earth to a kiloamp or so with a resistor or inductor in the star point or earthing transformer, and monitor it for fault detection.
 

Offline Nikos A.

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Re: Winding connection of transformers in power systems
« Reply #4 on: September 23, 2021, 06:29:37 pm »
What voltage range do you mean by HV?

I mean the 132KV/11KV step-down transformer. Is there any common winding connection for the primary and secondary of these transformers?
 

Offline f4eru

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Re: Winding connection of transformers in power systems
« Reply #5 on: September 23, 2021, 07:23:33 pm »
Some countries have weak resistive neutrals throughout distribution (11-30kV), so they can detect a fault, and let the neutral float from ground in case of P-E fault, and continue in degraded mode for a while, or even earth one faulty phase until the problem is resolved.

Others have the strategy to ground the neutral hard, then switch off in case of fault.
 
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